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United States Patent |
6,186,124
|
Stefanopoulou
,   et al.
|
February 13, 2001
|
System & method for controlling camshaft timing, air/fuel ratio, and
throttle position in an automotive internal combustion engine
Abstract
A system for controlling camshaft timing, air/fuel ratio and electronic
throttle position in an automotive internal combustion engine uses a
controller for operating a camshaft phaser, electronic throttle positioner
and fuel injectors. The controller determines camshaft timing,
steady-state electronic throttle position, steady-state fuel supply, and
compensatory transient electronic throttle position, and transient fuel
supply such that an engine operating with the present system has the
torque output characteristics matching a conventional engine having fixed
camshaft timing, but with lower fuel consumption and lower exhaust
emissions than a conventional engine.
Inventors:
|
Stefanopoulou; Anna (Ann Arbor, MI);
Jankovic; Mrdjan J (Birmingham, MI)
|
Assignee:
|
Ford Global Technologies, Inc. (Dearborn, MI)
|
Appl. No.:
|
302539 |
Filed:
|
April 30, 1999 |
Current U.S. Class: |
123/492; 123/322; 123/399 |
Intern'l Class: |
F02M 051/00 |
Field of Search: |
123/321,322,492,399,90.15,90.16,90.17,90.18
|
References Cited
U.S. Patent Documents
5101786 | Apr., 1992 | Kamio et al. | 123/399.
|
5143037 | Sep., 1992 | Swamoto | 123/90.
|
5168851 | Dec., 1992 | Itoyama et al. | 123/399.
|
5199403 | Apr., 1993 | Akazaki et al. | 123/90.
|
5220904 | Jun., 1993 | Miyashita et al. | 123/90.
|
5724927 | Mar., 1998 | Suzuki | 123/90.
|
6000375 | Dec., 1999 | Isobe | 123/399.
|
6006707 | Dec., 1999 | Ito | 123/90.
|
6062201 | May., 2000 | Nozawa et al. | 123/90.
|
Primary Examiner: Solis; Erick
Attorney, Agent or Firm: Drouillard; Jerome R.
Parent Case Text
This application is a divisional of Ser. No. 09/005,571, filed Jan. 12,
1998, now U.S. Pat. No. 6,006,725.
Claims
What is claimed is:
1. A method for controlling the camshaft timing, air/fuel ratio, and
electronic throttle position in an automotive internal combustion engine,
comprising:
determining a camshaft timing advance value for a camshaft which operates
cylinder intake and exhaust valves of the engine;
determining a steady-state position for an electronic air throttle;
determining a steady-state air/fuel ratio rate;
determining transient values for electronic air throttle position and
air/fuel ratio; and
determining a rich air/fuel ratio suitable for purging a lean NOx trap
based upon a quantity of fuel suitable for operating the engine at
approximately a stoichiometric air/fuel ratio, but with insufficient air,
so as to cause enrichment of the air and fuel mixture.
2. A method according to claim 1, wherein steady-state camshaft timing
advance and steady-state air/fuel ratio are based upon a sensed operating
position of a manually operable accelerator, as well as upon sensed engine
speed.
3. A method according to claim 1, wherein the determination of transient
electronic throttle position and transient fuel supply rate is based at
least in part upon the camshaft timing advance, and upon the steady-state
electronic throttle position.
4. A method according to claim 1, wherein transient electronic throttle
position is determined as a function of at least the time rate of change
of the camshaft timing.
5. A method according to claim 1, wherein transient electronic throttle
position is determined as a function of at least the time rate of change
of the camshaft timing and the instantaneous pressure within an air inlet
manifold of the engine.
6. A method according to claim 1, wherein transient electronic throttle
position comprises a steady state value and a correction value.
7. A method according to claim 1, wherein the transient air/fuel ratio is
determined so as to track and follow the filling and emptying of the
intake manifold with the result that fluctuations in output torque are
minimized.
Description
FIELD OF THE INVENTION
The present invention relates to a system and method for simultaneously
controlling transient camshaft timing, air/fuel ratio, and electronic
throttle position in an internal combustion engine.
BACKGROUND OF THE INVENTION
Lean-burn operation of spark-ignited internal combustion engines is
desirable because it improves fuel economy. By combining lean-burn and
variable cam timing (VCT) technologies in port fuel injected engines,
improvement in fuel economy of about 8 to 10% can be achieved. Moreover,
available data suggest that the feedgas emissions of a lean-burn VCT
engine are also improved. As used herein, the term "feedgas" means the
exhaust gas leaving the engine prior to any aftertreatment. And, the term
VCT refers to engine cylinder valve timing control of either intake and
exhaust valves or only exhaust valves.
Additional improvement in efficiency is possible by operating a direct
injection spark-ignited (DISI) engine in a very lean stratified-charge
mode. The present invention addresses the problem of scheduling camshaft
timing, air/fuel ratio, and electronic throttle position for lead
operation of both port-injection and DISI engines in order to achieve
optimum performance in terms of fuel efficiency and emissions as well as
the driveability or torque response of a conventional engine.
For the purposes of this specification, it is assumed that the engine is
equipped with an electronic throttle control (ETC) in which the vehicle
driver merely operates a potentiometer, with the actual throttle opening
being determined by the engine's electronic controller. Other sensors and
actuators used with conventional electronically controlled engines may be
employed with a system and method according to the present invention.
Performance of the present system during lean operation may benefit from a
universal exhaust gas oxygen (UEGO) sensor used instead of or in
conjunction with a heated exhaust gas oxygen(HEGO) sensor.
The additional degrees of freedom available in a lean-burn VCT engine make
the scheduling of camshaft timing, air/fuel ratio, and electronic throttle
position difficult. The method proposed in this specification is
structured to decouple driveability issues from the steady sate scheduling
of the ETC, camshaft timing and air/fuel ratio. The optimal steady state
schedules are obtained using the engine data of fuel consumption and HC,
CO, and NO.sub.x emissions at different cam and air-fuel values with
engine speed and braking torque held constant. Demanded torque at a given
engine speed can be achieved by many different combinations of throttle
position, camshaft timing, and air/fuel ratio. The present system and
method assures that, at a given torque demand, the steady state values of
camshaft timing and air/fuel ratio are optimal.
Transient operation of the ETC, the air/fuel ratio and camshaft timing must
be carefully managed in order to achieve torque response resembling a
conventional engine. Because the ETC (as an actuator) and fuel injectors
are much faster than the cam timing actuator, the following sequence is
employed for scheduling and dynamic transient compensation: (a) cam timing
command is as prescribed by optimal steady state schedules; (b) the ETC
command contains a component which is used to compensate for the cylinder
air-charge variation due to cam timing transients; and (c) the air/fuel
ratio command contains a dynamic component that matches the manifold
filling dynamics to avoid large torque excursions and driveability
problems. In general, when valve timing is moved from a more retarded
position to a more advanced position, the ETC must be placed in a more
closed position; conversely, when valve timing is moved from a more
advanced position to a more retarded position, the ETC must be placed in a
more open position.
One distinct feature of the proposed method is that the air/fuel scheduling
into the lead region is air-driven not fuel-driven. This makes the task of
simulating the driveability of a conventional engine much easier because
engine output torque is much more sensitive to fuel changes at constant
air than to air changes at constant fuel. For example, for a fixed flow of
fuel, changing the air flow from 20:1 lean to stoichiometric changes the
engine torque by about 6% to 8% as this only changes the efficiency of the
engine. On the other hand, for a fixed air flow, changing the fuel flow
from 20:1 lean to stoichiometric changes the torque by more than 30%.
To meet legislated tailpipe emission requirements, lean-burn engines must
be equipped with a "lean NO.sub.x trap" (LNT) to reduce the exhaust
concentration of the oxides of nitrogen (NO.sub.x). The LNT requires
periodic purging, which is accomplished by operating the engine at either
exact stoichiometry or at a rich air/fuel ratio for a period of time.
Changing the amount of fuel from lean to rich operation causes an increase
in torque which is not demanded by the driver, resulting in driveability
problems. The present air-driven method of operation avoids this problem
and allows purging of an LNT without causing torque variation.
During stratified operation of DISI engines the problem of fuel-driven
air-fuel control is even more pronounced and the benefits of the present
air-driven scheduling is more significant.
SUMMARY OF THE INVENTION
According to the present invention, a system for controlling the camshaft
timing, air/fuel ratio, and electronic throttle position in an automotive
internal combustion engine includes a camshaft phaser for controlling the
timing advance of a camshaft for operating cylinder intake and exhaust
valves of the engine, a throttle position sensor for sensing the position
of a manually operable accelerator and for producing an accelerator
position signal, and an engine speed sensor for sensing engine speed and
for producing an engine speed signal. The present system also includes an
electronic throttle positioner for setting an intake air throttle at a
commanded position, a plurality of fuel injectors for supplying fuel to
the engine, and a controller for operating the camshaft phaser, the
electronic throttle positioner, and the fuel injectors, with the
controller receiving the outputs of the accelerator position and engine
speed sensors, and with the controller determining camshaft timing
advance, steady-state and transient electronic throttle position, and fuel
supply.
According to another aspect of the present invention, the controller
determines an ETC setting appropriate to achieve a rich air/fuel ratio
suitable for purging a lean NO.sub.x trap based upon a quantity of fuel
suitable for operating the engine at approximately a stoichiometric
air/fuel ratio, but with excess air sufficient to cause enleanment of the
air and fuel mixture.
The engine controller of the present system operates the engine with
improved fuel economy by operating the fuel injectors to provide a
quantity of fuel suitable for operation at approximately a stoichiometric
air/fuel ratio, but with the camshaft phaser and the electronic throttle
being operated so as to provide an air charge having sufficient mass so as
to operate with a lean air/fuel ratio. This is essential to a fuel-driven
operating system, rather than the air-driven systems found in the prior
art.
The engine controller determines the transient electronic throttle position
as a function of at least the time rate of change of the camshaft timing,
and preferably, the instantaneous pressure within the engine's inlet
manifold.
According to another aspect of the present invention, a method for
controlling the camshaft timing, air/fuel ration, and electronic throttle
position in an automotive internal combustion engine comprises the steps
of determining camshaft timing advance value for a camshaft which operates
cylinder intake and exhaust valves of the engine, determining a
steady-state position for an electronic air throttle, determining a
steady-state fuel supply rate, and determining transient values for
electronic air throttle position, and fuel supply rate appropriate to
migrate to a desired rich or lean air/fuel ratio while allowing engine
torque output to closely approximate the torque output of the engine
without camshaft timing control.
The camshaft timing advance and steady-state air/fuel ratio are preferably
based upon a sensed operating position of a manually operable accelerator,
as well as upon sensed engine speed.
The present method may further comprise the step of determining a rich
air/fuel ratio suitable for purging a lean NO.sub.x trap based upon a
quantity of fuel suitable for operating the engine at approximately a
stoichiometric air/fuel ratio, but with a reduction in air sufficient to
cause enrichment of the air and fuel mixture. And, the present method may
further include the step of determining a lean air/fuel ratio for
operating the engine with increased fuel economy, followed by operation of
fuel injectors so as to provide a quantity of fuel suitable for operation
at approximately a stoichiometric air/fuel ratio, but with said camshaft
phaser and said electronic throttle being operated so as to provide a
sufficient air charge so as to operate with a lean air/fuel ratio.
According to another aspect of the present invention, transient air/fuel
ratio is determined so as to track and follow the filling and emptying of
the intake manifold, with the result that fluctuations in output torque
are minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of an engine having a control system
according to the present invention.
FIG. 2 is a plot of electronic throttle position as a function of
accelerator position.
FIG. 3a is a plot of camshaft position as a function of engine speed and
electronic throttle position.
FIG. 3b is a plot of air/fuel ratio as a function of engine speed and
electronic throttle position.
FIG. 4 is a plot of air charge as a function of engine speed and electronic
throttle position.
FIG. 5 is a plot of engine air flow as a function of the pressure ratio
across the throttle.
FIG. 6 is a plot of engine air flow as a function of throttle engine.
FIGS. 7A and 7B are plots of engine airflow as a function of engine speed
and camshaft position.
FIG. 8 is a plot of intake manifold reference pressure as a function of
throttle angle and engine speed.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In order to operate an engine according to the present invention, it is
necessary to know throttle position, as governed by an ETC, camshaft
operating position (also described as camshaft timing), and fuel injector
pulse width. Of course, knowing fuel injector pulse width and the
operating characteristics of the camshaft and electronic throttle, the
air/fuel ratio can be set to the desired rich, lean, or stoichiometric
point. In this manner the engine may be operated to achieve the best
emissions and fuel economy with a torque response which is the same as a
conventional engine operating at the stoichiometric air/fuel ratio and
fixed camshaft timing.
FIG. 1 illustrates an engine having a system according to the present
invention. Engine 10 is provided with fuel by means of fuel injectors 12
which are operated by engine controller 14. Engine controller 14 is of the
conventional variety known to those skilled in the art and as suggested by
this disclosure. Controller 14 also operates camshaft phaser 16 which
controls the timing advance of a camshaft which operates the cylinder
valves of engine 10.
Controller 14 receives an engine speed signal from speed sensor 18 as well
as a variety of other engine operating parameters from sensor 20, which
include such sensors as an intake manifold pressure sensor and other
sensors known to those skilled in the art and suggested by this
disclosure. A system according to the present invention further utilizes
electronic throttle positioner 22 which in effect provides a drive-by-wire
because the intake air throttle (not shown) is set at a position commanded
solely by controller 14; throttle position sensor 24 merely senses or
measures the position of a manually operable accelerator and produces an
accelerator position signal. In other words, the vehicle operator has no
direct link with the air throttle admitting air to the engine's intake
manifold.
FIG. 2 illustrates a plot of accelerator position AP versus electronic
throttle command .theta..sub.c. This is a calibratable function which will
give progressivity to the vehicle driver's accelerator command according
to the dictates of an engineer doing development work on a vehicle having
an engine and system according to the present invention. In other words,
the greater the slope of the plot, the more aggressively electronic
throttle positioner 22 will open the air throttle and in response the
driver's input. Simply state, .theta..sub.c is a measure of the torque
response demanded by the driver because the more aggressively the driver
depresses the accelerator pedal, the greater the driver's expectation of
engine response.
Having received a value .theta..sub.c from the plot of FIG. 2, which can be
merely a lookup table within controller 14, the engine controller moves to
FIGS. 3a and 3b, which is a three-dimensional plot, again in the form of a
lookup table having as independent variables .theta..sub.c from FIG. 2,
and engine speed, N. The plots of FIGS. 3a and 3b, which are determined
from engine mapping data, give camshaft position .GAMMA..sup.ref as well
as air/fuel ration, .alpha..sup.ref. The camshaft timing and air/fuel
ratio selected from three-dimensional lookup tables by controller 14 at
this step provide the camshaft timing and air/fuel required to achieve
optimal emissions and fuel economy at the driver's demanded torque. This
is the torque generated at the given accelerator pedal position and engine
speed by an engine operating with fixed camshaft timing and a
stoichiometric air/fuel ratio.
It is noted here that the camshaft position and air/fuel ratio vary to
provide the best emission control capability. These are optimal steady
state values dependent on engine speed and torque.
Knowing the steady-state camshaft position and air/fuel ratio, it is still
necessary to determine the required throttle position and fuel injector
pulse width.
To calculate the fuel charge required at stoichiometry, one need merely
take the cylinder air charge and divide by 14.64, which is the chemically
correct air/fuel ratio for a typical gasoline motor fuel. Thus, having
determined the desired fuel charge, it is necessary to calculate the
throttle angle required to achieve the lean air/fuel ratio .alpha..sup.ref
given the fuel flow previously calculated. As explained above, lean
operation is desired for reasons of fuel economy and emission control.
Also, we must determine an additional dynamic correction of throttle
position to avoid torque disturbances due to moving camshaft timing per
the schedule calculated for .GAMMA..sup.ref. The required cylinder airflow
is calculated as:
.phi..sub.cyl.sup.o =(desired fuel charge).times.(.alpha..sup.ref).
The values of .phi..sub.cyl.sup.o and N are sued to determine from engine
mapping data throttle angle .theta..sup.o to provide the desired air flow
for lean operation. For the purpose of purging an LNT, the engine must
operate with a rich air/fuel ratio, .alpha..sup.rich. In the same manner
.theta..sup.o was determined, a new air flow for rich operation can be
determined, along with a required ETC setting for rich operating,
.theta..sup.r. Thus, the required cam position, .GAMMA..sup.ref and
throttle positions .theta..sup.o and .theta..sup.r are stored as functions
.theta..sub.c and N in lookup tables in the memory of controller 14. These
lookup tables are used in real time to assure low emissions, good fuel
economy, and the driveablility of a conventional engine.
To assure that the engine's torque response is close to that of a
conventional engine, the steady state schedules described above are not
used directly for controlling cam timing, throttle position, and air/fuel
ratio. Instead, controller 14 adds a dynamic correction to ETC position
and air/fuel ratio command to avoid engine output torque disturbances. The
sequence of action of controller 14 may be summarized as follows.
1. The reference camshaft timing position, .GAMMA..sup.ref obtained from
the lookup table illustrated in FIG. 3a is used directly by camshaft
phaser 16 to control cam timing. In other words, the camshaft timing
command is not filtered. This is true because the response time of
camshaft phaser 16 generally slower than the response times of the ETC or
fuel injectors.
2. An additional throttle angle, .theta.*, needed to compensate for torque
disturbance caused by camshaft movement, is computed and added to
.theta..sup.o, the lean operation throttle setting, obtained from the
lookup table illustrated in FIG. 4. The sum of .theta..sup.o and .theta.*
is then used to command the ETC to a desired position.
3. Steady-state air/fuel ratio command, .alpha..sup.ref, is modified to
account for the dynamics the intake manifold. The modified value of
.alpha..sup.ref is then sued to determine the amount of fuel to be
delivered by fuel injectors 12.
4. For urging of an LNT, steps 2 and 3 above are repeated for
.theta..sup.r. Steps 1 and 4 are straightforward to implement; steps 2 and
3 will be explained below.
Controller 14 must calculate an additional throttle angle that when added
to the angle calculated above compensates for the torque disturbance
caused by moving camshaft to the desired position .GAMMA..sup.ref. This is
a dynamic correction which will only be applied while the camshaft phaser
16 is moving the camshaft to a new position. To make this correction,
controller 14 needs to know the mass airflow through the throttle body and
into the intake manifold. This is a well known function of pressure ratio
across the throttle valve and upstream temperature and pressure.
Graphically, this may be represented as two or more functions g.sub.1,
g.sub.2. FIGS. 5 and 6 illustrate these functions. FIG. 5 is a plot of
engine airflow at standard temperature and pressure as a function of
pressure ratio across the throttle, Pm/Pa.
FIG. 6 is a plot of engine airflow at standard temperature and pressure as
a function of throttle angle. The flow across the throttle,
.phi..sub..theta., equals g.sub.1.times.g.sub.2. The flow rate of air from
the intake manifold into the engine's cylinders can be represented by an
additional function comprising two parameters which are functions of
engine speed and camshaft timing position, plus intake manifold pressure
P.sub.m, which is a measured quantity.
FIGS. 7A and 7B illustrate parameters .alpha..sub.1 and .alpha..sub.2 which
are functions of camshaft position and engine speed. The values of
.alpha..sub.1 and .alpha..sub.2, which are readily available from engine
mapping, are stored in lookup tables within controller 14. Flow into the
cylinders is calculated as:
.phi..sub.cyl =.alpha..sub.1 P.sub.m +.alpha..sub.2.
Prior to making the final throttle correction, the reference manifold
pressure, P.sub.mref needs to be known. This is intake manifold pressure
corresponding to a stoichiometric fixed cam engine operating at a given
engine speed. This may be calculated from the perfect gas law or
tabulated.
FIG. 8 illustrates intake manifold pressure P.sub.mref as a function of
engine speed N and .theta..sup.o. Controller 14 now determines a throttle
correction by solving the following equation. Then, the transient throttle
correction is .theta.=.theta..sup.o +.theta.*.
.theta.*=(g.sub.2).sup.-1
{[(.differential..alpha..sub.1.differential..GAMMA..sub.cam) P.sub.m
+(.differential..alpha..sub.2
/.differential..GAMMA..sub.cam)].multidot.[d.GAMMA..sub.cam /dt]/[k.sub.m
g.sub.1 (P.sub.m).alpha..sub.1 ]+[g.sub.1 (P.sub.mref)/g.sub.1
(P.sub.m)]g.sub.2 (.theta..sup.o)}-.theta..sup.o
At this point, controller 14 has determined throttle position as
.theta.=.theta..sup.o +.theta.* for the ETC.
For step 3 above, an additional calculation is required, that is,
calculation of the fuel injector pulse width. Because fuel charge can be
changed faster than air, a change in fuel charge will cause an undesirable
air/fuel transient unless the fuel command is shaped until the air flow
catches up. This is accomplished by filtering the air/fuel ratio command
to account for the lag. The essential differential equation is given as
shown below:
de/dt=K.sub.m [g.sub.1 (P.sub.mref)g.sub.2 (.theta..sup.o)-g.sub.1
(P.sub.mref -e)g.sub.2 (.theta..sub.c)-.alpha..sub.1 (N,O)e]
Here, .alpha..sub.1 is evaluated without any camshaft advance. Then, the
correction factor applied to the air flow command is thus
.DELTA..phi..sub.cyl =.alpha..sub.1 (N,.theta.)e and .GAMMA..sub.AF which
is the commanded air/fuel ratio compensated for manifold filling dynamics
is given by the expression:
.GAMMA..sub.AF =14.64[1+.DELTA..phi..sub.cyl
/(.phi.cyl-.DELTA..phi..sub.cyl)]
In summary, controller 14 calculates .GAMMA..sup.ref, which is camshaft
position, .GAMMA..sub.AF which is the air/fuel ratio accounting for
manifold dynamics, and .theta.*, the throttle command accounting for
camshaft phaser dynamics. During transient operation, the camshaft
position, the ETC position, and the air/fuel ratio all change
continuously.
While the invention has been shown and described in its preferred
embodiments, it will be clear to those skilled in the arts to which it
pertains that many changes and modifications may be made thereto without
departing from the scope of the invention.
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